Meaning of RoomFilter with omitted account_data, ephemeral, state or timeline fields is unclear

[DMRobertson]

Link to problem area: https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3useruseridfilter, definition of RoomFilter

Issue

rooms says:

Name	Type	Description
rooms	[string]	A list of room IDs to include. If this list is absent then all rooms are included. This filter is applied before the filters in ephemeral, state, timeline or account_data

(emphasis mine).

The other fields do not describe the meaning of the filter if they are omitted.

Name	Type	Description
account_data	RoomEventFilter	The per user account data to include for rooms.
ephemeral	RoomEventFilter	The events that aren’t recorded in the room history, e.g. typing and receipts, to include for rooms.
state	StateFilter	The state events to include for rooms.
timeline	RoomEventFilter	The message and state update events to include for rooms.

Noticed in matrix-org/synapse#13648

[Benjamin-L]

All of the individual fields on {RoomEvent,State}Filter are optional, and the spec does describe their semantics when each field is omitted. I believe omitting any of these filters is equivalent to passing {} ({RoomEvent,State}Filter with all fields omitted). This is the behavior implemented by ruma's type definition (the #[serde(default...)] annotations cause it to deserialize json with that field missing using the default value for the field's type).